doc: update OS level support for AIX

Update OS level supprot for AIX.  Based on discussions
with the AIX team we've have decided that the minimum
support level should be 7.1. This is because support
for AIX 6.1 will likely end during the lifespan of
the upcoming Node version 8 release.

PR-URL: https://github.com/nodejs/node/pull/12235


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>